The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor, and particularly, it relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, a vehicle onboard camera, and a network camera.
In these years, in order to enhance convenience and improve security, a plurality of cameras is mounted on a vehicle. For example, in case of a vehicle equipped with a rear-view camera to take an image behind the vehicle, the view behind the vehicle is shown on a monitor when a driver drives the vehicle backward. Accordingly, the driver can safely drive the vehicle backward without contacting obstacles that cannot be directly seen by the driver. Such cameras for mounting on vehicles, i.e., so-called onboard cameras, are expected to increase in popularity.
Generally speaking, an onboard camera is often accommodated in a relatively small space such as in a back door, a front grille, a side mirror, and inside of the vehicle. For this reason, in addition to a small size, an imaging lens for mounting on the onboard camera is required to be suitable for a high resolution accompanied by an increased number of pixels of an imaging element and a wider angle to take in a broad range. However, it is difficult to achieve a small size and a high resolution as well as a wide imaging angle, while properly correcting aberrations. For example, when a size of an imaging lens is reduced, refractive power of individual lens tends to become stronger. Accordingly, it is difficult to satisfactorily correct aberrations. Therefore, upon actually designing an imaging lens, it is important to meet those requirements in a well-balanced manner.
As an imaging lens with a wide imaging angle, for example, Patent Reference has disclosed a conventional imaging lens. The conventional imaging lens includes a negative first lens that has a shape of a meniscus lens directing a convex surface thereof to the object side, a second lens having a biconcave shape, a third lens having a biconvex shape, an aperture stop, a fourth lens having a biconvex shape, and a negative fifth lens that has a shape of a meniscus lens directing a concave surface thereof to the object side, arranged in this order from an object side.
According to the conventional imaging lens disclosed in Patent Reference, the third lens and the fifth lens in the configuration are formed of a high-dispersion material. Accordingly, it is possible to correct a field curvature and a chromatic aberration of magnification. Further, the second lens is formed in a shape of a biconcave lens near an optical axis thereof to achieve a wide angle, thereby increasing negative refractive power.
Patent Reference: Japanese Patent Application Publication No. 2011-107593
According to the conventional imaging lens described in Patent Reference, although the number of lenses that compose the imaging lens is as few as five, an imaging angle of view is wide and it is possible to relatively satisfactorily correct aberrations. In the conventional imaging lens, however, a total length of the whole lens system tends to become long relative to a focal length. Accordingly, it is difficult to achieve a small size. In these days, there remain issues to achieve both downsizing of the imaging lens and satisfactory correcting aberration. Here, such an issue is not a problem specific to the imaging lens for mounting on onboard cameras. Rather, it is a common problem for an imaging lens to accommodate in a relatively small camera such as cellular phones, digital still cameras, portable information terminals, security cameras, and network cameras.
In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that has wide imaging angle of view and can satisfactorily correct aberrations in spite of a small size thereof.
Further objects and advantages of the present invention will be apparent from the following description of the present invention.